1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, more particularly relates to a nonvolatile memory device comprised of a channel-forming region comprising a surface portion of a semiconductor substrate, a well, semiconductor film or other on which a tunnel film, an insulation film, and a gate electrode are laminated in order from the lower layer.
2. Description of the Related Art
In an MNOS (Metal-Nitride-Oxide Semiconductor) type or MONOS (Metal-Oxide-Nitride-Oxide Semiconductor) type nonvolatile memory device (hereinafter also referred to as a xe2x80x9cMIOS typexe2x80x9d), the gate electrode is comprised of a single layer and thus has a simpler element structure in comparison with a floating gate (hereinafter described as an FG) type nonvolatile memory device in which the gate electrode has a two-layer structure.
FIG. 11 is a sectional view of a MONOS type nonvolatile memory device. As shown in this figure, the MONOS type nonvolatile memory device 6 is structured of a channel-forming region 11a of a semiconductor substrate 11 on which a tunnel film 12 made of a silicon oxide film or an oxi-nitride film, etc., an insulation film 13 comprising a silicon nitride film 13a and a silicon oxide film 13b as the upper layer thereof, and a gate electrode 14 are laminated in order from the lower layer. The term xe2x80x9cchannel-forming regionxe2x80x9d in the present invention means a region where a channel through which electrons or positive holes pass is formed. Various forms exist, for example, a surface part of a semiconductor substrate per se, a surface part of a well formed in the surface of a semiconductor substrate and a semiconductor film.
In the tunnel film 12 in a MONOS type nonvolatile memory device, the thickness thereof is important in terms of determining the characteristics of a memory element. For example, mentioning one example, in Cheng Wang, xe2x80x9cHot Carrier Design Consideration for MOS Devices and Circuitsxe2x80x9d, p. 219, the thickness of the tunnel film 12 is set within a range of T=1.5 nm to 2.0 nm or so. The same is true for an MNOS type nonvolatile memory device in which the insulation film 13 is made of a single layer of silicon nitride film 13a. 
The thickness of the tunnel film 12 is a thin one of about ⅕ the thickness of the tunnel oxide film in an FG type nonvolatile memory device.
Further, in these MIOS type nonvolatile memory devices, since the thickness of the tunnel film 12 is thin as described above, injection of charges at a lower gate voltage in comparison with an FG type nonvolatile memory device is possible and the write characteristic is excellent.
However, a MIOS type nonvolatile memory device was inferior in its data retention characteristic to an FG type nonvolatile memory device. In general, the period of guarantee of the data retention in an FG type nonvolatile memory device is 10 years at 125xc2x0 C. As opposed to this, the period of guarantee of the data retention in a MIOS type nonvolatile memory device is 10 years at 85xc2x0 C.
Further, in an MIOS type nonvolatile memory device, the write characteristics at a low voltage were excellent as mentioned above, but there was a problem of a read disturbance, that is, charges were accumulated in the insulation film at a low gate voltage at the time of a read operation and erroneous writing occurred. For this reason, by adopting a two-transistor structure in which selection transistors are individually provided for every memory element, occurrence of erroneous writing due to read disturbances has been prevented. Accordingly, in a conventional MIOS type nonvolatile memory device, in comparison with an FG type nonvolatile memory device in which it is not necessary to provide a selection transistor for every memory element, the cell area became 1.5 times or more the size.
The present invention was made in consideration with such an actual circumstance and has as an object thereof to provide a nonvolatile semiconductor memory device in which the data retention characteristic and the resistance to read disturbances are improved. Further, another object of the present invention is to realize a so-called one-transistor cell structure without providing a selection transistor by improving the resistance to read disturbances, preferably, further, by forming the memory element as an enhancement type.
To achieve the above object, the present invention provides a nonvolatile semiconductor memory device comprised of a channel-forming region of a semiconductor on which a tunnel film, an insulation film, and a gate electrode are laminated in order from the lower layer, wherein the thickness of the tunnel film is within a range where charges in the semiconductor substrate directly tunnel through the tunnel film and is 2.2 nm or more, thereby forming a one-transistor cell structure.
In the nonvolatile semiconductor memory device, although the thickness of the tunnel film is set within the range where the charges in the channel-forming region directly tunnel through the tunnel film, charges are injected into the insulation film by the mechanism of modified F-N (Fowler-Nordheim) tunneling similar to the conventional one. Further, since the thickness of the tunnel film is greater in comparison with a conventional MIOS type nonvolatile memory device, i.e., 2.2 nm or more, this tunnel film acts as a barrier, it becomes difficult for the charges injected into the insulation film to leak into the semiconductor substrate, and the data retention characteristic is improved. Along with this, it becomes difficult for the charges to be injected into the insulation film at a low gate voltage at the time of a read operation.
Further, in the nonvolatile memory device, the characteristic feature resides in that the insulation film has a higher content of silicon than the stoichiometric ratio Si3N4 (Si:N=3:4).
By constituting the insulation film by silicon nitride having a high content of silicon as described above, a trap density of charges in the insulation film is increased, and the conductivity of the insulation film become s high. For this reason, when voltage is applied to the gate electrode, a stronger electric field is applied with respect to the tunnel film, thus the increase of the application voltage at the time of a write/erase operation can be suppressed low by making the tunnel film thicker than the conventional thickness.
On the other hand, in order to make it difficult for a read disturbance to arise, in a MONOS type nonvolatile memory device, the top oxide thickness may be made thin within a range where the amount of the required change of the threshold voltage at the time of a write operation is obtained. In this case, the insulation film of the present invention is constituted by a nitride film on the tunnel film and a top oxide film on the nitride film. Another characteristic feature resides in that the thickness of the tunnel film is set to 2.2 nm or more. Although the absolute thickness of the top oxide film changes according to the film quality thereof, from the viewpoint of the amount of transition of the carriers, the top oxide film is set to almost same thickness as the thickness of the tunnel film or the thickness lower than this. Defining this in more detail, the thickness of the top oxide film is set to a thickness that the amount of transition of the carriers passing through the top oxide film is almost equal to or larger than the amount of transition of the carrier passing through the tunnel film.
Preferably, the top oxide film is set to a smaller thickness than the thickness of the tunnel film.
As mentioned above, if the tunnel oxide film is made thick, it becomes difficult to inject charges into the insulation film by a low gate voltage at the time of a read operation. In addition to this, If the top oxide film is made thin, the thickness is adjusted so as to balance intensities of the electric fields applied to the tunnel film in xe2x80x9cerasedxe2x80x9d state and the top oxide film in xe2x80x9cwrittenxe2x80x9d state at the time of reading data, and It becomes difficult for a read disturbance to occur. Namely, the injected charges are no longer blocked by the thick top oxide film, the rise of the threshold voltage in xe2x80x9cerasedxe2x80x9d state is suppressed, and erroneous writing at the time of a read operation Is prevented.
By these structures, the resistance to read disturbances is improved, so the selection transistors become unnecessary at the time of a read operation and it becomes possible to realize a so-called one-transistor cell.
Further, if the tunnel film is made thicker than the prescribed value (3.4 nm), the write and erase operations become possible within the range of the enhancement type, which is advantageous for a one-transistor cell structure.